Exfoliated Molybdenum Disulfide Encapsulated in a Metal Organic Framework for Enhanced Photocatalytic Hydrogen Evolution

An exfoliated MoS2 encapsulated into metal-organic frameworks (MOFs) was fabricated as a promising noble-metal-free photocatalyst for hydrogen production under visible light irradiation. The as-synthesized samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller (BET) surface analysis. It is well known that bulk MoS2 is unsuitable for photocatalysis due to its inadequate reduction and oxidation capabilities. However, exfoliated MoS2 exhibits a direct band gap of 2.8 eV due to quantum confinement, which enables it to possess suitable band positions and retain a good visible-light absorption ability. As a result, it is considered to be an encouraging candidate for photocatalytic applications. Encapsulating exfoliated MoS2 into MOF demonstrates an improved visible light absorption ability compared to pure MOF, and the highest hydrogen production rate that the encapsulated exfoliated MoS2 could reach was 68.4 μmol h-1g-1, which was much higher than that of pure MOF. With a suitable band structure and improved light-harvesting ability, exfoliated MoS2@MOF could be a potential photocatalyst for hydrogen production.


Introduction
The increase in demand for clean and renewable energy has motivated new studies for developing novel techniques for hydrogen and oxygen production via water splitting [1][2][3][4][5].Photocatalysis has been shown to be an attractive process for generating H 2 and O 2 , using natural sunlight as an irradiation source [6][7][8][9].Lately, transition-metal oxides (TMOs) and transition metal dichalcogenides (TMDs) have been demonstrated to be effective and efficient catalytic materials [10][11][12].In order to use visible light, which occupies about 50% of all solar energy, photocatalysts should meet the following requirements in order to effectively convert solar energy into H 2 via photocatalytic water splitting: the conduction band minimum (CBM) must be more negative than the hydrogen evolution potential 0 V vs. normal hydrogen electrode (NHE) (pH = 0) and the valence band maximum (VBM) must be more positive than the water oxidation level +1.23 V vs. NHE (pH = 0), respectively, and all chemical and structural properties must be stable during the photocatalytic reaction [2,4].The band structure of the semiconductor component and its redox potentials for water splitting are the keys to discovering suitable candidates for photocatalytic hydrogen evolution [4].For TMOs, WO 3 can act as a steady photocatalyst for O 2 evolution under visible light illumination together with a suitable electron acceptor.
However, the conduction band of the material is located in a more positive position compared to the water reduction potential, and as a consequence, WO 3 cannot reduce H + to H 2 directly [13].For TMDs, CdSe and CdS possess suitable band structures for visible light-driven water splitting, but they are not stable under the photocatalytic reaction conditions due to their photocorrosion properties (poor photoinduced stability) [5,14].Therefore, the conventional TMOs and TMDs cannot effectively split water under visible light illumination without any modifications.
MOFs have drawn substantial attention in research for their adjustable pore size, large pore volume, high specific surface area, and highly ordered porous structures.Previous studies have mostly focused on MOFs for sensing, drug delivery, batteries and selective catalysis.Their application as photocatalysts, however, has not been thoroughly reported, and their photocatalytic ability is limited as MOFs lack the capacity to absorb visible light [15,16].While it is well known that bulk MoS 2 is unsuitable for photocatalytic applications owing to its inadequate reduction and oxidation capabilities, exfoliated MoS 2 shows a direct band gap of 1.9~3.9eV due to quantum confinement.This enables MoS 2 to possess suitable band positions for absorbing visible-light [17-21], and thus MoS 2 is considered to be a great candidate for photocatalytic applications.Therefore, combining MOFs with a visible-light-active photocatalyst, such as encapsulating TMDs into MOFs to produce core-shell structures, offers combined properties that may lead to synergistic functionalities [14,22].Recently, increasing attention has been centered on applying MOFs, especially the Zeolitic Imidazolate Framework (ZIF-8) structure, to carbon capture, sensing, drug delivery, batteries and selective catalysis, however, their application as photocatalysts to address both environmental issues and the energy crisis has not been thoroughly reported [23][24][25][26].
As one of the metal-organic frameworks (MOFs), the zeolitic imidazolate framework-8 (ZIF-8) is constructed by Zn(II) and 2-methylimidazole ligands, and possesses a high specific surface area and outstanding thermal and chemical stability [26].Although it is extensively applied in adsorption, catalysis, and gas separation [23,26], the photocatalytic properties of ZIF-8 have been inadequately studied.In Zeng et al.'s study, CdS@ZIF-8 with core-shell structures were synthesized by a two-step method.CdS NPs were first stabilized by polyvinylpyrrolidone (PVP) and then ZIF-8 shells were developed on the surface of the CdS cores [14].The core-shell CdS@ZIF-8 structures were used for photocatalytic H 2 evolution from formic acid and revealed better selectivity for H 2 production compared to the pure CdS nanoparticles, however, the H 2 production rate of the as-prepared samples was limited.This study focused on developing an exfoliated MoS 2 @MOF composite to evaluate its potential for photocatalytic hydrogen evolution.The new photocatalyst showed increased absorption in visible light range with an improved hydrogen production rate compared with pure MOF.

Morphology
The exfoliated MoS 2 @ZIF-8 (denoted as MZ) was synthesized by a two-step method.First, the exfoliated MoS 2 (denoted as EM) was prepared by sonicating the MoS 2 /methanol dispersion in an ultrasonic bath.Next, the EM was added in the process of preparing ZIF-8 to obtain MZ.The morphology of the as-prepared MZ was characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses.Figure 1a,b shows that the MZ comprised of irregular agglomerates, with the particle size ranging from 10 to 30 µm. Figure 1c,d are the EDS elemental mapping results of MZ for the C, N, O, Zn, Mo, and S elements.The Zn, C, and N elements were well dispersed in the MZ particles, while Mo and S elements were clearly encapsulated in the ZIF-8 shell [14].From the elemental distribution results of the O element, it can be seen that ZnO rarely existed in the MZ without further calcination.
Figure 2b exhibits the N2 adsorption and desorption isotherms of the MZ and the ZIF-8, respectively.Figure 2c shows the pore size distribution of MZ and ZIF-8.The Brunauer-Emmett-Teller (BET) surface area of ZIF-8 was as high as 1,313.8m 2 /g while the BET surface area of MZ reduced to 955.3 m 2 /g, but similarities were observed in the surface pore characteristics of the assynthesized samples.Both curves of the prepared samples could be classified as having Type I isotherm characteristics that suggest their microporous nature.Nonetheless, the existence of large pores suggested that the isotherms changed to Type IV for values of P/P0 larger than 0.85, which was possibly attributed to intra-aggregate voids [29].
The specific surface areas and average pore diameters of the as-prepared materials were examined based on N2 adsorption and desorption results (Table 1).XPS analysis was carried out to investigate the chemical states and surface chemical compositions of MZ (Figure 2).The XPS spectra of N1s, O1s, Zn2p, and their simulated decomposition peaks are shown in Figure 2d-f, respectively.For the N1s spectra, the peak appearing at 398.8 eV corresponded to the sp 2 -hybridized nitrogen (C=N-C) group [24].The N1s and C1s peaks could be assigned to the existence of ZIF-8 that was   002), ( 112), ( 022), (013), and (222) lattice planes of ZIF-8, respectively [28].
Figure 2b exhibits the N 2 adsorption and desorption isotherms of the MZ and the ZIF-8, respectively.Figure 2c shows the pore size distribution of MZ and ZIF-8.The Brunauer-Emmett-Teller (BET) surface area of ZIF-8 was as high as 1313.8m 2 /g while the BET surface area of MZ reduced to 955.3 m 2 /g, but similarities were observed in the surface pore characteristics of the as-synthesized samples.Both curves of the prepared samples could be classified as having Type I isotherm characteristics that suggest their microporous nature.Nonetheless, the existence of large pores suggested that the isotherms changed to Type IV for values of P/P 0 larger than 0.85, which was possibly attributed to intra-aggregate voids [29].
The specific surface areas and average pore diameters of the as-prepared materials were examined based on N 2 adsorption and desorption results (Table 1).XPS analysis was carried out to investigate the chemical states and surface chemical compositions of MZ (Figure 2).The XPS spectra of N1s, O1s, Zn2p, and their simulated decomposition peaks are shown in Figure 2d-f, respectively.For the N1s spectra, the peak appearing at 398.8 eV corresponded to the sp 2 -hybridized nitrogen (C=N-C) group [24].The N1s and C1s peaks could be assigned to the existence of ZIF-8 that was prepared by 2-methylimidazole [22].The Zn2p 1/2 and Zn2p 3/2 peaks (Figure 2f) at the binding energies of 1045.5 and 1022.1 eV were closer to the metallic phase of Zn, which meant that the Zn element mainly stayed in the Zn 0 state.The main peak in the O1s region at 530.3 eV was assigned to carbonates (Figure 2f) [30].

Optical Properties and Photocatalytic Hydrogen Evolution
Generally, bulk MoS 2 (band gap of 1.2 eV) is unsuitable to act as a photocatalyst, due to its limited reduction and oxidation capability which is required for activating the photocatalytic process.Nonetheless, exfoliated MoS 2 displays a direct band gap of 1.89~3.96eV stemming from quantum confinement.In Huang et al.'s work, monolayer MoS 2 quantum dots (MQDs) were prepared via a hydrothermal method and exhibited a 3.96 eV of direct band gap compared to monolayer MoS 2 NSs (1.89 eV) [31].By using scanning tunneling microscopy (STM), another study reported that chemical vapor deposition (CVD)-prepared single-layer MoS 2 possessed a band gap of 2.40 eV [32].Based on the self-consistent GW 0 (scGW 0 ) calculations, Shi et al. predicted that the quasiparticle (QP) band structures of monolayer MoS 2 exhibited a direct band gap energy of 2.80 eV [33].This offered the exfoliated MoS 2 the suitable band positions needed to retain the desired visible-light absorption ability.As a result, the exfoliated MoS 2 was considered to be a good candidate for photocatalytic applications.
The optical absorption of ZIF-8, EM, and MZ was examined by UV-Vis absorption spectra (Figure 3).The absorption edge for ZIF-8 (Figure 3a) at ~250 nm without significant absorption in the visible light region suggested a band gap of 5.15 eV.The absorption edge of EM (Figure 3b) was at ~390 nm and showed a limited visible light absorption ability with a band gap of 2.84 eV.However, for EM (a band gap of 3.41 eV), the absorption edge appeared at ~415 nm (Figure 3c), which indicated that EM was effective in the visible light region.The possible mechanism is illustrated in Figure 3d.Methanol (CH 3 OH) was used as a sacrificial electron donor to provide protons with electrons in the experiments.CH 3 OH was initially oxidized by the excited holes, which generated protons (H + ) with a hydroxyalkyl radical intermediate (•CH 2 OH).Then •CH 2 OH further produced H + and electrons that subsequently formed H 2 .The photocatalytic performance of MZ was studied by examining the photocatalytic hydrogen production under visible light with a Xe lamp irradiation (AM 1.5 G, 100 mW cm -2 ).The hydrogen evolution rate for the reported ZIF-8 was 0.01 µmol h -1 g -1 , and no activity was shown for bulk MoS 2 .As shown in Figure 3e, with the coexistence of EM, MZ gave an improved average hydrogen evolution rate of 29.9 µmol h -1 g -1 .

Synthesis
The process of synthesizing exfoliated MoS2 was as follows.100 mg of MoS2 bulk powder (purchased from Sigma-Aldrich, St. Louis, USA) was first immersed in 100 mL of methanol (purchased from Sigma-Aldrich, St. Louis, USA).Then exfoliation was achieved by sonicating the dispersion in an ultrasonic bath for 1 h and then probe-sonicating for another 1 h.The bath temperature during sonication was kept below 277 K by immersing the sample in an ice water bath.The top two-thirds of the exfoliated MoS2 were collected for further experiments.

Synthesis
The process of synthesizing exfoliated MoS 2 was as follows.100 mg of MoS 2 bulk powder (purchased from Sigma-Aldrich, St. Louis, MO, USA) was first immersed in 100 mL of methanol (purchased from Sigma-Aldrich, St. Louis, MO, USA).Then exfoliation was achieved by sonicating the dispersion in an ultrasonic bath for 1 h and then probe-sonicating for another 1 h.The bath temperature during sonication was kept below 277 K by immersing the sample in an ice water bath.The top two-thirds of the exfoliated MoS 2 were collected for further experiments.
The process of synthesizing MoS 2 @ZIF8 was as follows.Zinc nitrate hexahydrate (Zn(NO 3 ) 2 •6H 2 O), 2.47 mmol) was first added to the exfoliated MoS 2 methanolic solution (25 mL).Next, the resulting solution was added to 25 mL of 2-methylimidazole (19.85 mmol), kept at room temperature and stirred for 12 h.Lastly, the dispersion was centrifuged three times at 10,000 rpm for 5 minutes and vacuum-dried to obtain MZ.

Characterization
Structural and elemental characterization of the as-prepared samples were analyzed on a Hitachi (S-4800) scanning electron microscope (Hitachi, Tokyo, Japan).The X-ray powder diffraction investigation was carried out on a Bruker D8 Discover X-ray diffractometer (Bruker, Billerica, MA, USA) equipped with a scintillation counter and a Cu k-alpha radiation (0.15418 nm) reflection mode.Renishaw Raman microscope was used for studying the Raman spectroscopy with an excitation wavelength of 1064.X-ray photoelectron spectroscopy spectra were obtained using a PerkinElemer PHI 5440 ESCA System (PHI-5440, PerkinElemer, Waltham, MA, USA) with monochromatic Al Kα radiation functioning as the X-ray source.The C1s peak located at 285 eV was used as a reference point in this study.ASAP2020 (Micromeritics, Norcross, GA, USA) Brunauer-Emmett-Teller N 2 adsorption-desorption was used to obtain the specific surface area.The N 2 adsorption-desorption tests were conducted at 77 K using a Quantachrome Autosorb gas-sorption system (Quantachrome, Boynton Beach, FL, USA).All the samples were degassed at 200 • C for 2 h before the examinations.The BET method was used to estimate the sample-specific surface area.

Measurements
Compressed N 2 was the carrier gas used for examining the performance of hydrogen evolution.The N 2 carrier gas was regulated by two mass flow controllers continuously moving through a bubbler which contained 5 vol.% methanol mixed in de-ionized water to create a water-methanol vapor system.Methanol in the mixture acted as a sacrificial electron donor to provide H + with electrons.The volume fraction of water (2.2%) and methanol (0.6%) in the mixed gas at 20 • C was evaluated through the saturation vapor pressure-temperature profile, which was balanced with the N 2 carrier gas.The mixed gas was then transferred into a photoreactor.Next, 25 mg samples were prepared on a glass fiber filter and placed in the middle of the tube reactor.A solar light simulator (Newport Oriel, Sol1ATM 94021A, Newport, Irvine, CA, USA) equipped with a Xe lamp and an Air Mass 1.5 global filter (AM 1.5 G, 100 mW cm -2 ) with a normal incidence compared to the sample served as the photo-excitation source.Before the illumination, the reactor was initially cleaned by the N 2 at a flow rate of 200 ml/min for 1 h and next at 2 mL/min for 1 h.The flow rate of carrier gas was maintained at 2 mL/min during the illumination and the sample was recorded every 15 min by an automated valve to control the gas chromatography (GC, Agilent 7890A, Agilent, Santa Clara, CA, USA).The GC in this study used a thermal conductivity detector (TCD).

Conclusions
In summary, a facile and effective method to synthesize exfoliated MoS 2 @ZIF-8 hybrids with improved photocatalytic activity in hydrogen production was demonstrated.Encapsulating exfoliated MoS 2 into ZIF-8 could maintain the desired porous structural characteristics of MOFs and the large specific surface area (955.3 m 2 /g for MZ) that are both critical for photocatalysis.The obtained hybrid photocatalyst exhibited an enhanced absorption (~415 nm) in visible light range compared to pure ZIF-8 (~250 nm).As a result, the as-prepared MoS 2 @ZIF-8 hybrids possessed a more desirable band gap structure (3.41 eV) than ZIF-8 (5.15 eV), which lead to an enhanced efficiency for photocatalysis.In comparison with the pristine ZIF-8, the MZ exhibited substantially enhanced photocatalytic activity for hydrogen production at 29.9 µmol h -1 g -1 under visible-light irradiation.The improved photocatalytic performance could be ascribed to the larger specific surface area, suitable band structure, and improved visible light-harvesting ability resulting from the hybrid structure.

Catalysts 2018, 8 ,
x FOR PEER REVIEW 4 of 9 prepared by 2-methylimidazole[22].The Zn2p1/2 and Zn2p3/2 peaks (Figure2f) at the binding energies of 1045.5 and 1022.1 eV were closer to the metallic phase of Zn, which meant that the Zn element mainly stayed in the Zn 0 state.The main peak in the O1s region at 530.3 eV was assigned to carbonates (Figure2f)[30].

Figure 3 .
Figure 3. UV-vis absorption spectra and Tauc's plot (inset) for band gap energy determination of (a) ZIF-8, (b) exfoliated MoS2 (EM), and (c) MZ; (d) schematic illustration of the energy band structure of the as-prepared samples; (e) photocatalytic hydrogen production rate collected for MZ.

Figure 3 .
Figure 3. UV-vis absorption spectra and Tauc's plot (inset) for band gap energy determination of (a) ZIF-8, (b) exfoliated MoS 2 (EM), and (c) MZ; (d) schematic illustration of the energy band structure of the as-prepared samples; (e) photocatalytic hydrogen production rate collected for MZ.

Table 1 .
Surface properties of ZIF-8 and as-synthesized MZ.

Table 1 .
Surface properties of ZIF-8 and as-synthesized MZ.